I would say to get another copy of the CD so that she can still hear the songs, and just explain to her that you have to sell the curriculum for 2 reasons:
1.) She's outgrowing it and
2.) You already told a lady that she could buy it

I guess you could call/email the woman who you sold it to and ask her if you can finish it out and then send it to her, in case she isn't planning on using it right away....